Fortress and later: rpath complaint for libGrid3D.dylib on macOS

Environment

• OS Version: link provided for macOS Big Sur (11.7.7-x86_64) specifically, may affect other versions of macOS
• Source or binary build? binary build of 6.8.0

Description

• Expected behavior: brew linkage --test --strict gz-gui6 should pass without errors
• Actual behavior: it complains that libGrid3D.dylib has missing rpath

I see from this cmake code that libGrid3D is installed as a copy of libGridConfig, but it seems that the rpath information is lost.

Steps to reproduce

1. brew install gz-gui6
2. brew linkage --test --strict gz-gui6

Output

From https://build.osrfoundation.org/job/generic-release-homebrew_triggered_bottle_builder/label=osx_bigsur/1201/console

==> brew linkage --cached --test --strict osrf/simulation/ignition-gui6
==> FAILED
Files with missing rpath:
  /usr/local/Cellar/ignition-gui6/6.8.0_2/lib/ign-gui-6/plugins/libGrid3D.dylib